The photosensitive lithographic printing plates which are commercially available at the present time, are classified into two groups, that is, outside type and inside type. The outside type photosensitive lithographic printing plate, which are disclosed, for example, in Japanese Patent Application Publication (Kokoku) No. 47-25481, is prepared by coating a surface-treated aluminium plate surface with a water-soluble photosensitive diazo resin. The photosensitive resin layer of the outside type photosensitive lithographic printing plate is made insoluble by exposure to light. Therefore, when a non-exposed portion of the photosensitive resin layer which is soluble in water or a diluted soap aqueous solution, is removed by water, a lithographic printing plate having desired images is obtained. However, this type of lithographic printing plate is disadvantageous in its poor ink-receiving property and poor durability in printing. In order to eliminate the above-mentioned disadvantages, it is necessary to coat the image surfaces of the lithographic printing plate with an emulsion type lacquer which enhances the ink-receiving property and the durability of the images.
The inside type photosensitive lithographic printing plate has a photosensitive resin layer which contains a water-insoluble reinforcing resin. In this case, the resultant lithographic printing plate has an excellent durability in printing. However, the water-insoluble reinforcing resin in the photosensitive layer causes the photosensitive resin layer to be not capable of being developed with water or an aqueous neutral liquid. Accordingly, the photosensitive layer needs to be developed with a special developing liquid, such as an organic solvent, alkali solution or an aqueous solution of a surface active agent. This type of development is disclosed, for example, in Japanese Patent Application Publication (Kokoku) No. 37-11558. However, the above-mentioned special developing liquid is not preferable in view point of the labor hygiene, the safty of operation and environmental pollution due to the discharged waste liquid from the developing process.
In order to eliminate the above-mentioned problems, Japanese Patent Application Laying-open (Kokai) No. 52-6202 provided a photosensitive lithographic printing plate which comprises a supporting layer comprising a substrate and a hydrophilic surface, and a photosensitive layer which is formed on the hydrophilic surface of the supporting layer and which comprises a mixture of:
(A) a photosensitive diazo resin which has been prepared by pre-condensation polymerizing an aromatic diazonium compound and an aldehyde compound in the presence of an acid catalyst, and by condensation-polymerizing the pre-condensation product on a phenol compound, and; PA1 (B) a water-soluble acrylic polymer containing, as a polymerization component, at least one member selected from acrylic acid and .alpha.- and .beta.-substituted acrylic acid derivatives. PA1 a supporting layer comprising a substrate and having a hydrophilic surface, and; PA1 a photosensitive layer which is formed on the hydrophilic surface of the supporting layer and PA1 (A) a photosensitive diazo resin modified with a phenol compound, which resin has been prepared by pre-condensation polymerizing an aromatic diazonium compound an aldehyde compound in the presence of an acid catalyst, by blending the pre-condensation product with a phenol compound, and by complete condensation polymerizing the blend, and; PA1 (B) a water-soluble acrylic polymer containing, as a polymerizing component, at least one member selected from the group consisting of acrylic acid and .alpha.- and .beta.-substituted acrylic acid derivatives;
The above-mentioned Japanese Laid Open Patent Application also discloses a hydrophilic layer which is located between the substrate and the photosensitive layer so as to form the hydrophilic surface of the supporting layer by the surface of the hydrophilic layer. The hydrophilic layer thus formed is effective for considerably improving the durability and developing property of the resultant photosensitive lithographic printing plate. However, the improved durability of the above-mentioned type of lithographic printing plate is still unsatisfactory from the view point of practical industrial use.
Under the circumstances, it is strongly desired to further improve the durability in printing of the conventional lithographic printing plate. Also, it is desired to improve the printing property of the lithographic printing plate so as not to soil the background of the printed images by the printing plate per se. Moreover, it is desirable that the photosensitive lithographic printing plate have a constant developing property which causes the printing plate to be completely developed under a constant developing condition. Furthermore, it is desirable that the hydrophilic layer and the photosensitive layer can be easily prepared by a simple coating operation.